1. Field of the Invention
The present invention relates to a polymer electrolyte medium and a direct methanol fuel cell.
2. Description of the Related Art
As a polymer electrolyte medium (proton conductive film) of a direct methanol fuel cell (DMFC), a perfluoroalkylsulfonic acid film such as a fluorine-based ion exchange film is conventionally used. An example of such a conventionally used film is a product of DuPont, known by its tradename, Nafion.
However, the conventional proton conducting film entails the following drawbacks. That is, the main chain of its polymer contains a fluorocarbon structure, and the film has an affinity for methanol. For this reason, when the fuel cell is in operation, the film is dissolved to deteriorate. Further, the proton conductive film is deteriorated by radical spices created by the electrode reaction while the fuel cell is in operation as well as the electric field. Due to the deterioration of the proton conductive film caused by these factors, crossover of methanol is generated, causing not only a decrease in the methanol use efficiency, but also a decrease in output of the fuel cell. Furthermore, the fluorine-based ion exchange film known as its tradename of Nafion by DuPont is expensive, causing an increase in the production cost of the fuel cell.
On the other hand, Jpn. Pat. Appln. KOKAI Publications No. 2000-223135 and No. 2003-36864 each disclose an improved solid polymer electrolyte film used for a fuel cell. The solid polymer electrolyte film is formed of a main chain consisting of a co-polymer of a fluorocarbon-based vinyl monomer and hydrocarbon-based vinyl monomer, and a hydrocarbon-based side chain including a sulfonic group, and the side chain contains an α-methylstylene group.
However, even in the solid polymer electrolyte film, the main chain contains a fluorocarbon structure, and the film has an affinity for methanol. For this reason, while the fuel cell is in operation, the film is dissolved to deteriorate. Further, the film is deteriorated by radical spices created by the electrode reaction while the fuel cell is in operation as well as the electric field. As a result, crossover of methanol is generated, causing a decrease in output of the fuel cell.